The present invention relates generally to fuel delivery systems. More particularly, the invention relates to an electronic circuit for controlling the pump in a fuel injection system.
In a conventional fuel injection delivery system fuel is pumped continuously in a closed fluid circuit from the gas tank, to the fuel rail, back to the gas tank. When the engine is running, the fuel pump operates at full speed, with all unused fuel being recirculated to the tank.
Some automotive engineers believe that it would be desirable to eliminate the continuous recirculation of fuel and hence eliminate the fuel return to tank. This poses the problem of how to control the flow rate of fuel pumped from the tank to the fuel rail if no return to tank is provided. The flow rate is a function of engine RPM and load and thus cannot be considered constant. What is needed, but not available today, is a practical, mass-producible means for controlling the flow rate produced by the fuel pump. Conventional pump control techniques do not meet these requirements and tend to produce severe radio frequency interference (RFI), making them unsuitable for vehicular use.
The present invention overcomes prior deficiencies in fuel delivery systems by providing a pressure responsive motor drive circuit for operating a fluid delivery pump. The invention is well suited to vehicular mass production and overcomes many of the previous problems with radio frequency interference.
According to the invention, a pressure transducer provides an electrical signal in response to a sensed pressure condition within the fuel delivery system. An amplifier circuit, coupled to the pressure transducer, amplifies the electrical signal to produce an amplified signal which is summed with a predetermined DC offset to produce an offset signal. An oscillator circuit, preferably relaxation oscillator, produces an alternating signal of a predetermined frequency, preferably a sawtooth waveform having an exponentially curved rising edge. A comparator circuit is coupled to the oscillator circuit and to the amplifier circuit for producing a pulse train which changes state each time the alternating signal rises above and falls below the level of the offset signal. The comparator thereby produces a pulse signal. A power transistor circuit is coupled to the comparator circuit and provides a current to drive the pump motor. In this fashion the pump motor speed is controlled by the sensed pressure condition in the fluid delivery system and the need for a return to tank is eliminated.
Radio frequency interference is minimized by a power conditioning circuit having an energy storing means such as a capacitor for receiving electrical energy through a power supply transmission line from the vehicular power source. Typically this transmission line spans the length of the vehicle, from vehicle alternator/battery to fuel tank-mounted fuel pump. The energy storage means is coupled to the power transistor circuit to supply energy from which the pulse width modulated current is produced. The energy storage means is of a capacity sufficient to minimize rapid voltage changes over the transmission line and thereby reduce radio frequency radiation by the transmission line. The energy storage means provides sufficient dampening so that the energy being conveyed over the transmission line has a comparatively rounded or slowly fluctuating waveform with few high frequency harmonics. By minimizing high frequency harmonics, radio frequency radiation by the transmission line is minimized.
To further reduce radio frequency interference, the power transistor circuit includes a complementary reactance coupled across the motor terminals to substantially null the inductance of the motor. The combined impedance of the motor and complementary reactance is substantially resistive. This reduces ringing overshoot of the motor drive current which would otherwise potentially cause radio interference in the AM broadcast band.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.